1. Field of the Invention
The present invention relates generally to methods and apparatus for stretch breaking fibers. More particularly, the present invention involves stretching and breaking bundles of relatively brittle fibers using methods and apparatus that reduce the amount of small fiber filaments which are produced during the stretch breaking process.
2. Description of Related Art
Composite materials are well known and widely used in applications where a high strength and light weight material is required. Composite materials are typically composed of fibers that are embedded in a polymer resin matrix. Glass and carbon are two of the most popular fiber materials. Epoxy and phenolic resins are two of the most popular resin matrix materials. The fibers used in composite materials can be arranged in a wide variety of configurations depending upon the desired final properties of the composite. For example, fibers can be randomly oriented in the resin matrix or they can be woven into a wide variety of fabric patterns.
In many applications, multiple fibers are combined to form yarn that is woven to form fabric which is impregnated with resin and cured to form the final composite. In many situations, it is desirable to use yarn which contains continuous unbroken fibers. However, there are a number of situations where yarn containing broken fibers or filaments are desirable. For instance, yarn containing discontinuous fibers is useful in situations where the composite material is formed into complex shapes. Fabric made from yarn containing discontinuous fibers is easier to form around tight bends than fabric made using continuous fibers.
A common method for forming yarn with discontinuous fibers is referred to as xe2x80x9cstretch breakingxe2x80x9d. Typical stretch breaking methods involve coating a fiber bundle with various viscous lubricants and stretching the bundle until the individual fibers break or fracture into multiple fragments. In some methods, the fiber bundle is subjected to breaker bars during stretching to facilitate fracturing of the fibers. The resulting fractured yarn is used in the same manner as unbroken yarn except that it must be handled more carefully to prevent the yarn from falling apart. The viscous lubricant is designed, among other things, to help keep the bundle of fractured fiber together. Exemplary stretch breaking methods are described in U.S. Pat. Nos. 4,759,985 and 4,825,635.
Although the existing methods for stretch breaking fibers are well suited for their intended purpose, there is a continuing need to improve upon such methods. For example, many of the stretch break methods produce a large number of relatively short (ie. less than 30 microns long) fiber fragments. The generation of a relatively large number of short fiber fragments reduces the strength of the final composite material. In addition, the short fibers tend to separate out from the fractured bundle during stretching and during subsequent handling. This not only causes potential pollution problems, but also results in loss of mass from the yarn. Further, the amount of viscous lubricants used to coat the fibers must be carefully controlled. In most applications, the amount of lubricant is kept below one percent by weight of the total yarn bundle weight. If too much lubricant is used, the adhesion of the fractured fibers to the resin matrix can be adversely affected. If too little lubricant is used, the broken bundle will lack the desired cohesiveness and may not be further processed (i.e., the bundle falls apart).
In accordance with the present invention, a method and apparatus are provided for stretch breaking fibers wherein the formation of relatively short filaments is reduced and the need for viscous lubricants is eliminated. The present invention is based upon the discovery that the shock waves generated during fracturing of stretched fibers cause the formation of a high proportion of relatively short (i.e., less than 30 microns long) filaments. It was discovered that the application of a sufficient amount of dampening fluid, such as water, to the stretched fiber bundle provides sufficient dampening of the shock waves to reduce the amount of relatively short filaments which are formed.
The present invention also includes the addition of compatible coatings to the stretch broken bundles to increase bundle cohesiveness and ensure bundle integrity as they are handled subsequent to stretch breaking.
One aspect of the present invention involves apparatus for stretch breaking fibers to provide bundles of fractured filaments having different lengths. The apparatus includes a bundle anchoring device which anchors the bundle or xe2x80x9ctowxe2x80x9d of fibers at a first end of the bundle. The apparatus further includes a bundle pulling device which pulls on the bundle of fibers at a second end to produce a stretched bundle extending between the first and second ends of the bundle. The amount of stretching provided by the bundle pulling device is sufficient to break the fibers to form fractured filaments having different lengths. The apparatus further includes a fluid applicator which applies a dampening fluid to the bundle of fibers. A sufficient amount of dampening fluid is applied to the bundle in order to provide dampening of the shock waves generated along substantially the entire length of the stretched bundle. It was discovered that saturating substantially the entire length of the stretched bundle of fibers provides sufficient dampening of the shock waves to reduce the amount of relatively short fiber filaments formed during fracturing of the fibers.
As a feature of the present invention, water is a preferred dampening fluid, because it is extremely inexpensive and can be removed easily by evaporation. Further, it was discovered that the tendency of the stretch-broken tows or yarn to fall apart could be reduced by leaving a small amount of water in the fiber bundle. The damp fractured bundle is then treated with compatible coatings or sizing materials to increase the cohesiveness of the bundle sufficiently so that the integrity of the bundle is not compromised during subsequent handling.
Another aspect of the present invention involves methods for stretch breaking fibers to provide bundles of fractured filaments having different lengths. The method includes the step of anchoring a bundle of fibers at a first end with an anchoring device, such as a pair of rollers. In a second step, a pulling device, such as a pair of rollers, is provided for pulling on the bundle of fibers to provide sufficient stretching to fracture the fibers to form filaments having different lengths. As a feature of the invention, the method includes applying a sufficient amount of a dampening fluid to the stretched bundle of fibers to provide dampening of the shock waves along substantially the entire length of the stretched bundle. In a final step of the method, the dampening fluid is removed from the bundle after formation of the fractured filaments and/or a compatible coating is applied to increase the cohesiveness of the bundle to provide a stable product which can be processed further without falling apart.
The present invention is also directed to the bundles of stretch-broken fibers formed using the above-summarized apparatus and method. In addition, the invention covers the textiles and composite materials that include bundles of stretch-broken fibers as described above.
The present invention is an improvement over existing methods and apparatus in that stretch breaking of fibers is provided wherein the number of relatively short fractured filaments is reduced and wherein the resulting fractured fiber bundles are free of viscous lubricants.
The above discussed and many other features and attendant advantages of the present invention will become better understood by reference to the following detailed description when taken in conjunction with the accompanying drawings.